1. Technical Field of the Invention
The present invention relates to a dimmer such as a rheostat for carrying out dimming control for a load of an illuminating system through pulse width modulation control (PWM control).
2. Related Art
A related dimmer for regulating a quantity of light of a conventional illuminating lamp is shown in FIG. 5, for example. A dimmer 1 shown in FIG. 5 comprises a PWM circuit 3 for outputting a PWM control signal based on pulse width modulation (which will be hereinafter referred to as PWM). An output of the PWM circuit 3 is regulated corresponding to a set value represented by a dimming volume 2. The PWM control signal is generated by the PWM circuit 3 and is sent to a driving circuit 4, and a driving operation of a switch circuit 5 using a semiconductor switch such as a transistor is controlled by the driving circuit 4. The reference numeral 6 denotes an operating switch for bringing the PWM circuit 3 into an operation state. With this structure, a magnitude of a load current supplied from a battery 7 to a load 8 of an illuminating system is changed corresponding to a switching operation of the switch circuit 5 so that a quantity of light emission of the load 8 of the illuminating system can be dimmed and controlled by the PWM.
Moreover, a cut-off circuit 9 is connected to the driving circuit 4 of which operation is controlled in response to the PWM control signal. The operation of the driving circuit 4 can be cut off by the cut-off circuit 9. In addition to a basic circuit portion including the PWM circuit 3, the driving circuit 4, the switch circuit 5 and the cut-off circuit 9, furthermore, an overcurrent detecting circuit 10 is provided to avoid such a situation that an overcurrent generated by an overload or a short-circuit flows to the switch circuit 5 and an overheat protecting circuit 11 is provided to prevent the overheat of the switch circuit 5, thereby controlling an element temperature to have a constant value or less. When an overcurrent flowing to the load 8 and an overheat state of the switch circuit 5 are detected and an output signal of the overcurrent detecting circuit 10 or the overheat protecting circuit 11 is input to the cut-off circuit 9, the operation of the driving circuit 4 is cut off by the cut-off circuit 9.
Moreover, a lamp soft start circuit 13 is provided between the dimming volume 2 and the PWM circuit 3. A set value signal variably regulated by the dimming volume 2 is caused to slowly rise by the lamp soft start circuit 13 and is then input to the PWM circuit 3 so that a rush current can be prevented from flowing into the load 8 when the operating switch 6 is turned ON.
In the related dimmer 1, however, additional circuit portions including the overcurrent detecting circuit 10, the overheat protecting circuit 11 and the lamp soft start circuit 13 are generally provided on the outside of each of basic circuit portions including the PWM circuit 3, the driving circuit 4, the switch circuit 5 and the cut-off circuit 9 in order to maintain versatility. Therefore, there are the following problems.
(1) The number of parts required for constructing the dimmer 1 is increased and a whole outside dimension is increased due to assembly of each circuit portion.
(2) In the existing art, it is substantially impossible to further enhance the detection precision of the overcurrent detecting circuit 10. For this reason, in such a circuit structure that the existent overcurrent detecting circuit 10 is applied to the dimmer 1, an overcurrent often flows to an element of the switch circuit 5 so that a stress of the element is increased. In some cases, therefore, the switch circuit 5 cannot be protected for a desirable constant period.
In consideration of the circumstances, it is an object of the invention to provide a dimmer having a simple circuit structure, a small size and high reliability.
A dimmer according to the invention comprises:
a semiconductor switch which controls power supply from a power source to a load of an illuminating system;
a driver which outputs a driving control signal to the semiconductor switch;
a reference voltage generator which generates a reference voltage having a voltage characteristic almost equivalent to a voltage characteristic between terminals of the semiconductor switch;
a overcurrent detector which detects an overcurrent of the semiconductor switch based on a difference between the voltage between the terminals of the semiconductor switch and the reference voltage;
a switch controller which controls an ON/OFF state of the driver in response to the overcurrent detected by the overcurrent detector to control the semiconductor switch; and
a modulation signal generator which generates a pulse width modulation signal to control the driving control signal,
wherein at least the semiconductor switch, the reference voltage generator and the overcurrent detector are formed on a same chip.
Furthermore, it is possible to use the switch controller counts a time required for ON/OFF controlling the semiconductor switch or a number of times of ON/OFF control and supplies a cut-off signal to the driver when a count value exceeds a preset value.
Moreover, it is possible to use the reference voltage generator includes a reference voltage generating circuit having a second semiconductor switch connected in parallel with the semiconductor switch, the second semiconductor switch is connected in series with a resistor provided outside the chip.
As described above, the semiconductor switch, the reference voltage generator and the overcurrent detector are formed on the same chip to constitute the switching device, thereby controlling supply of power to the load of the illuminating system. Consequently, a circuit structure can be simplified and the size of an apparatus can be reduced. In addition, it is possible to implement a dimmer capable of enhancing the precision of overcurrent detection and having a small size and high reliability.